Internal combustion engines, whether Diesel cycle, Otto cycle, two or three stroke, comprise at least a sliding element such as a piston ring.
In this respect, the piston ring acts in the sealing of the space between the cylinder liner and the body of the piston, isolating the combustion chamber from the remaining internal components of the engines. The piston ring is disposed radially upon the base of the body of the piston, preventing the combustion gases from escaping from the combustion chamber in the direction of the crankcase and preventing the oil of the engine from penetrating into the combustion chamber.
Some internal combustion engines, principally engines operating with a Diesel cycle, work under high loads. Other examples may be found in high performance petrol engines. Independently of the examples which may be cited there is a tendency for engines to operate at high speed and high power, with reduced clearances and, consequently, to be subject to severe tribological behaviour. Such conditions are naturally more demanding on the mechanical components thereof. In this respect, the rings utilised in these engines of high power or performance require low friction, high hardness and high wear resistance.
Additionally, it is important to emphasise that the environmental impact of internal combustion engines, when linked to the need for high performance and durability, results, in a general manner, in the requirement to work with closer tolerances, this being, naturally, translated into the utilisation of increasingly thin layers of lubricating oils. As shall be seen hereinafter, the present invention has been specifically developed to offer a solution, the excellent performance whereof flows from the utilisation of predominantly less viscous oils.
The piston rings of the state of the art generally comprise coatings of hard amorphous carbon, also known as DLC (diamond like carbon) or hydrogenated nanostructure of DLC free of hydrogen, as a solution to achieve low friction and high wear resistance.
In general the solutions of the state of the art apply the coating of DLC in a composition incorporating sp3 and sp2 bonds. Nevertheless it should be noted that, by virtue of the high dimensional stability of the sp3 bonds, the hardness thereof exceeds those encountered with the sp2 bonds. For this reason diverse solutions of the state of the art make use of a DLC layer of sp3 (diamond type), coated by a thin layer of sp2 (graphite type).
For a better understanding, the sp2 layer, by virtue of being very soft and lubricant, is utilized to permit the bedding in of the ring within the cylinder liner and preventing the sp3 layer from entering directly into contact with the cylinder liner. Such concern is extremely valid by virtue of the fact that, in addition to the possibility that the sp3 layer may score the cylinder liner, the concentration of stresses present in the sp3 layer is very high such that it may generate disastrous consequences, both for the cylinder liner, scoring it, and for the coating, having the possibility of the occurrence of cracks and the propagation thereof, consequently leading to the diminution of the working life of the engine.
The patent document WO2010133633 reveals a piston ring provided with a DLC coating of the ta-C type having a thickness exceeding 10 micrometers and having at least a residual stress gradient to endow durability and low friction upon the component. It should be noted that the high internal stresses inherent in the deposition of a DLC coating result in the necessity of finding solutions such as those of this document. However, even reducing the residual stress, this solution does not fully resolve the problem, by virtue of the fact that the contact interface of the ring with the liner continues to occur by means of a profile not ensuring, for example, that the cylinder liner does not suffer the wear typical of the solutions of the state of the art.
The document US 2013/0140776 describes a piston ring provided with a DLC coating of the ta-C type having a thickness also exceeding 10 micrometers, wherein the coating contains a finishing layer having a thickness from 1 to 3 micrometers, wherein the quantity of sp3 decreases to values of less than 40% with a view to ensuring that a greater quantity of sp2 is present in this second layer for bedding in the ring within the cylinder.
Whilst the solutions of the state of the art are concerned with ensuring low friction and high durability, all the solutions found require an interface or presence of elements promoting the bedding in, such as a softer layer in the sliding region of the ring, or the addition of metals promoting such effects. In addition, there are alternatives in the state of the art to maintain the functional integrity of the coating, in view of the high stress, through the reduction of the internal stresses whereto the coating is naturally subject by virtue of the high internal stresses and high hardness.
Independently of the efforts revealed by the documents of the prior art, a solution has still not been found generating excellent results in the finished product, that is to say a piston ring not requiring bedding in within in the respective cylinder liner and which, at the same time, works with very small clearances.
Consequently, it is necessary to find a sliding element, such as a piston ring, comprising a coating of hard amorphous carbon of the a-C type provided with a roughness profile capable of promoting a reduction of the friction at the sliding interface thereof, together with diminishing the wear.